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======Exercise 36: Safer Strings======
I've already introduced you to the Better String library in Exercise 26
when we made devpkg. This exercise is designed to get you into using
bstring from now on, why C's strings are an incredibly bad idea, and
then have you change the liblcthw code to use bstring.
======Why C Strings Were A Horrible Idea======
When people talk about problems with C, it's concept of a "string" is
one of the top flaws. You've been using these extensively, and I've
talked about the kinds of flaws they have, but there's not much that
explains exactly why C strings are flawed and always will be. I'll try
to explain that right now, but part of my explanation will just be that
after decades of using C's strings there's enough evidence that they
are just a bad idea.
It is impossible to confirm that any given C string is valid:
* A C string is invalid if it does not end in '\0'.
* Any loop that processes an invalid C string will loop infinitely
(or, just buffer overflow).
* C strings do not have a known length, so the only way to check if
it's terminated correctly is to loop through it.
* Therefore, it is not possible to validate a C string without
possibly looping infinitely.
This is simple logic. You can't write a loop that checks if a C string
is valid because invalid C strings cause loops to never terminate.
That's it, and the only solution is to include the size. Once you know
the size you can avoid the infinite loop problem. If you look at the
two functions I showed you from Exercise 27 you can see this:
void copy(char to[], char from[])
{
int i = 0;
// while loop will not end if from isn't '\0' terminated
while((to[i] = from[i]) != '\0') {
++i;
}
}
int safercopy(int from_len, char *from, int to_len, char *to)
{
int i = 0;
int max = from_len > to_len - 1 ? to_len - 1 : from_len;
// to_len must have at least 1 byte
if(from_len < 0 || to_len <= 0) return -1;
for(i = 0; i < max; i++) {
to[i] = from[i];
}
to[to_len - 1] = '\0';
return i;
}
Imagine you want to add a check to the copy function to confirm that
the from string is valid. How would you do that? Why you'd write a loop
that checked that the string ended in '\0'. Oh wait, if the string
doesn't end in '\0' then how does the checking loop end? It doesn't.
Checkmate.
No matter what you do, you can't check that a C string is valid without
knowing the length of the underlying storage, and in this case the
safercopy includes those lengths. This function doesn't have the same
problem as it's loops will always terminate, even if you lie to it
about the size, you still have to give it a finite size.
What the Better String library does is create a struct that always
includes the length of the string's storage. Because the length is
always available to a bstring then all of its operations can be safer.
The loops will terminate, the contents can be validated, and it will
not have this major flaw. The bstring library also comes with a ton of
operations you need with strings, like splitting, formatting,
searching, and they are most likely done right and safer.
There could be flaws in bstring, but it's been around a long time so
those are probably minimal. They still find flaws in glibc so what's a
programmer to do right?
======Using bstrlib======
There's quite a few improved string libraries, but I like bstrlib
because it fits in one file for the basics and has most of the stuff
you need to deal with strings. You've already used it a bit, so in this
exercise you'll go get the two files bstrlib.c and bstrlib.h from the
Better String
Here's me doing this in the liblcthw project directory:
$ mkdir bstrlib
$ cd bstrlib/
$ unzip ~/Downloads/bstrlib-05122010.zip
======Archive: /Users/zedshaw/Downloads/bstrlib-05122010.zip======
...
$ ls
bsafe.c bstraux.c bstrlib.h bstrwrap.h license.txt
test.cpp
bsafe.h bstraux.h bstrlib.txt cpptest.cpp porting.txt
testaux.c
bstest.c bstrlib.c bstrwrap.cpp gpl.txt security.txt
$ mv bstrlib.h bstrlib.c ../src/lcthw/
$ cd ../
$ rm -rf bstrlib
# make the edits
$ vim src/lcthw/bstrlib.c
$ make clean all
...
$
On line 14 you seem me edit the bstrlib.c file to move it to a new
location and to fix a bug on OSX. Here's the diff:
25c25
< #include "bstrlib.h"
---
> #include
2759c2759
< #ifdef __GNUC__
---
> #if defined(__GNUC__) && !defined(__APPLE__)
That is, change the include to be , and then fix one
of the ifdef at line 2759.
======Learning The Library======
This exercise is short and simply getting you ready for the remaining
exercises that use the library. In the next two exercises I'll use
bstrlib.c to create a Hashmap data structure.
You should now get familiar with this library by reading the header
file, the implementations, and then write a tests/bstr_tests.c that
tests out the following functions:
bfromcstr
Create a bstring from a C style constant.
blk2bstr
Same but give the length of the buffer.
bstrcpy
Copy a bstring.
bassign
Set one bstring to another.
bassigncstr
Set a bstring to a C string's contents.
bassignblk
Set a bstring to a C string but give the length.
bdestroy
Destroy a bstring.
bconcat
Concatenate one bstring onto another.
bstricmp
Compare two bstrings returning the same result as strcmp.
biseq
Tests if two bstrings are equal.
binstr
Tells if one bstring is in another.
bfindreplace
Find one bstring in another then replace it with a third.
bsplit
How to split a bstring into a bstrList.
bformat
Doing a format string, super handy.
blength
Getting the length of a bstring.
bdata
Getting the data from a bstring.
bchar
Getting a char from a bstring.
Your test should try out all of these operations, and a few more that
you find interesting from the header file. Make sure to run the test
under valgrind to make sure you use the memory correctly.
Copyright (C) 2010 Zed. A. Shaw
Credits